The present invention relates to a thin film transistor. In particular, it relates to such a thin film transistor with a poly-crystalline silicon layer, and still more particularly, it relates to such a transistor of a double gate type which is suitable for use in an amplifier.
A thin film transistor is conventionally used for a switching element for a liquid crystal display element. An amplifier which uses a thin film transistor is also described in "Characteristics of Op amps using polycrystalline silicon thin film transistors" a paper given at the Autumn conference No. C549 organized by the Institute of Electronics, Information, and Communication in Japan, in 1990.
In order to provide an amplifier having a thin film transistor with excellent yield rate production and excellent operational characteristics, the curve showing the relationship between drain voltage (V.sub.D) and drain current (I.sub.D) of a transistor must have a preferable relationship. In other words, the drain current (I.sub.D) is preferably constant irrespective of changes in the drain voltage (V.sub.D). That characteristic was conventionally obtained by using a pentode vacuum tube.
However, a prior thin film transistor has V.sub.D to I.sub.D characteristics as shown in the curve (A) in FIG. 10, in which the drain current I.sub.D increases when the drain voltage V.sub.D exceeds a predetermined value because of an avalanche effect. That predetermined value is for instance around 8 V. The avalanche effect appears in particular when a transistor is produced in a poly-crystalline silicon layer, as compared with what happens when the transistor is produced in a single crystal silicon layer. Those characteristic are not preferable as an amplifier.
Another requirement for a thin film transistor for use in an amplifier is that threshold voltage, which is defined as gate voltage to turn ON or turn OFF a transistor, is controllable, for obtaining excellent amplification.
However, the control of the threshold voltage of a thin film transistor is conventionally impossible.